Glacial Lake Iroquois was a prehistoric proglacial lake

Proglacial lake

In geology, a proglacial lake is a lake formed either by the damming action of a moraine or ice dam during the retreat of a melting glacier, or by meltwater trapped against an ice sheet due to isostatic depression of the crust around the ice...

 that existed at the end of the last ice age

Ice age

An ice age or, more precisely, glacial age, is a generic geological period of long-term reduction in the temperature of the Earth's surface and atmosphere, resulting in the presence or expansion of continental ice sheets, polar ice sheets and alpine glaciers...

 approximately 13,000 years ago.
The lake was essentially an enlargement of the present Lake Ontario

Lake Ontario

Lake Ontario is one of the five Great Lakes of North America. It is bounded on the north and southwest by the Canadian province of Ontario, and on the south by the American state of New York. Ontario, Canada's most populous province, was named for the lake. In the Wyandot language, ontarío means...

 that formed because the St. Lawrence River downstream from the lake was blocked by the ice sheet near the present Thousand Islands

Thousand Islands

The Thousand Islands is the name of an archipelago of islands that straddle the Canada-U.S. border in the Saint Lawrence River as it emerges from the northeast corner of Lake Ontario. They stretch for about downstream from Kingston, Ontario. The Canadian islands are in the province of Ontario, the...

. The level of the lake was approximately 30 m (~100 ft) above the present level of Lake Ontario.

The lake drained to the southeast, through a channel passing near present day Rome, New York

Rome, New York

Rome is a city in Oneida County, New York, United States. It is located in north-central or "upstate" New York. The population was 44,797 at the 2010 census. It is in New York's 24th congressional district. In 1758, British forces began construction of Fort Stanwix at this strategic location, but...

.